In the infrared imaging and tracking system, infrared grey image of dim small target is disturbed by the background and noise that results in low SNR. A new method of infrared dim small target detection based on mathe- maticalmorphology is designed. First the noise and the air background of original image are supp ressed by Top-Hat transform ofmathematicalmorphology, and the target is segmented by open processing, then the little target signal in IR can be detected with tract filter. The results show that thismethod can significantly enhance SNR of target, and is adap table to real2time image p rocessing.